1. Field of the Invention
The invention relates to methods and apparatus for producing optical crystals. In particular, the invention relates to a method and an apparatus for annealing optical crystals, particularly optical lithography fluoride crystals for transmitting below 250-nm UV light.
2. Background Art
Optical crystals are commonly grown using the Stockbarger-Bridgman method. In the Stockbarger-Bridgman method, the optical crystals are grown in a vertical furnace by moving molten crystal material through a temperature gradient zone in the furnace. The method is further explained below with reference to FIGS. 1A and 1B.
FIG. 1A shows a vertical furnace 1 having an upper zone 2 and a lower zone 3. Heating jackets 4, 5 are provided in the upper and lower zones 2, 3, respectively. The heating jackets 4, 5 are operated such that a temperature gradient zone 6 is created between the upper and lower zones 2, 3. At the start of the growth process, a crucible 7 containing a crystal raw material F is mounted in the upper zone 2. The crystal raw material F is melted by heat from the heating jacket 4. After melting the crystal raw material F, the crucible 7 is lowered into the lower zone 3, as shown in FIG. 1B. As the crucible 7 passes from the upper zone 2 into the lower zone 3, the molten material M goes through the temperature gradient zone 6. On passing through the temperature gradient zone 6, the temperature transition inside the molten material M creates a crystallization front CF. The crystallization front CF propagates inside the crucible 7, within the molten material M, as long as the crucible 7 continues to move downwardly.
Crystals grown using the method described above are exposed to sharp localized cooling as they are translated through the temperature gradient zone into the lower zone. Sharp localized cooling induces permanent thermal strain (or stress) in the crystals, which can result in unacceptably elevated values in birefringence of the crystals. To reduce the permanent thermal strain in the crystal, the crystal is annealed in the lower zone of the growth furnace. The annealing cycle includes re-heating the crystal to a temperature below the melting temperature of the crystal, holding the crystal at this temperature until the thermal strain induced in the crystal by the sharp localized cooling is dissipated, and then slowly cooling the crystal to a temperature below which any strain due to additional cooling to room temperature will result only in temporary strain in the crystal.
The duration of the annealing cycle depends on the volume of the crystal. As the volume of the crystal increases, the ability to completely anneal the crystal inside the growth furnace such that the birefringence of the crystal meets the specification reduces. For instance, exposure systems in microlithography processes require optical crystals, mainly fluoride crystals, with birefringence values of 3 nm/cm or lower. To meet such stringent specifications for large-volume crystals, the growth furnace would have to be tied up for extended times, which would have a great impact on the ability to meet market demands. Therefore, the current practice is to anneal the crystal for a relatively short time in the growth furnace. The birefringence of the crystal is then measured. If the crystal has an unacceptably high birefringence value, the crystal is further annealed in a separate furnace from the growth furnace. This process is typically referred to as post-annealing.
A typical annealing furnace is a vertical furnace in which a vertical stack of individual hermetically-sealed containers can be supported during post-annealing. The furnace includes heaters for creating a desired temperature profile inside the furnace. In operation, the crystals to be annealed are loaded into the sealed containers, and the sealed containers are loaded into the annealing furnace. A vacuum, inert, or fluorinating atmosphere may be provided inside the sealed containers. The annealing process starts by heating the crystals to a temperature below the melting point of the crystals. The crystals are held at this temperature for a predetermined length of time before being slowly cooled to room temperature. Typically, the heaters used in the process are circumferential heaters, which are arranged in the furnace so as to circumscribe the individual containers. In addition, heaters or thermal insulators can be placed at the top and bottom of the stack of containers.
The annealing cycle can be relatively short if the crystals in the stack have small diameters, e.g., less than 150 mm. This is because the path of conduction from the circumference of the crystals, where the heat is applied, to the center of the crystals is relatively short. Thus, the heating rates from room temperature to annealing temperature and the cooling rates from annealing temperature to room temperature can be relatively high. However, as the diameters of the crystals increase, the path of conduction from the circumference of the crystals to the center of the crystals increases. As a result, the time required to complete the annealing process such that a desired birefringence level in the crystal is achieved also increases. Currently, there are demands for optical fluoride crystals with diameters of 300 mm or greater. Therefore, a process of annealing multiple large-diameter (crystal blank disk diameter>150 mm, preferably ≧250 mm, more preferably ≧300 mm) crystals within a reasonable time frame is desirable.